Boulder: Flood risk elevated this spring

'How the creeks behave won't be as predictable as it was in the past'

What: Boulder City Council briefing on flood recovery and emergency preparedness

When: 4 to 5:30 p.m. Tuesday

Where: Boulder Municipal Building, 1777 Broadway, council chambers

If You Go

What: Flood Preparedness and Recovery Open Houses

When: 6 to 9 p.m. March 31, presentation from 7 to 7:30 p.m.

Where: Municipal Building, 1777 Broadway, lobby and council chambers

When: 5 to 8 p.m. April 2, presentation from 6 to 6:30 p.m.

Where: Casey Middle School, 1301 High St., cafeteria and auditorium

Info: For more information on flood recovery efforts and how to prepare yourself for future flooding, go to boulderfloodinfo.net.

EDITOR'S NOTE: The city of Boulder changed the time of the March 31 flood preparedness meeting. The meeting is now 6 to 9 p.m., with the presentation from 7 to 7:30 p.m.

Elevated water tables, higher than normal snowpack and waterways that still have sediment and debris in them mean it will take less water than normal to cause new flooding this spring and summer, and it's much harder to predict where the water will go.

That's the finding of the Threat, Hazard Identification and Risk Assessment prepared by the city-county Boulder Office of Emergency Management. The Boulder City Council will get a briefing on the hazards report and city mitigation efforts on Tuesday from OEM Director Mike Chard before the regular council meeting.

"There is a lot of sediment in the creeks, so when we get thunderstorms, which we do pretty routinely, how the creeks behave won't be as predictable as it was in the past," said Jeff Arthur, Boulder director of public works for utilities. "Thunderstorms where they are predicting a small amount of water that would not have caught our attention in the past, we're going to be paying more attention."

Boulder and Boulder County were on similar alert the summer after the Four Mile Fire, but the danger now stretches across all 14 of the city's drainageways.

City contractors are working hard to return drainageways to their pre-flood condition so they can carry more water, with crews removing sediment and debris and work scheduled to start this week on restoring flood control devices like the Boulder Drops, which stabilize flow and guide water so it's less likely to open new channels.

However, the city's work won't restore entire waterways.

"We're working toward getting the creeks back to pre-flood capacity, but that's only on city-owned land or on city easements," Boulder Engineering Project Manager Kurt Bauer said. "There is a lot of stuff west of the city that, if we get a big rainfall, could come back down. We'll find that out this year."

Spring run-off season, which starts in April, primarily affects Boulder Creek and South Boulder Creek. The city's other waterways originate in the foothills and are less affected by snowmelt, but could be affected by thunderstorms.

The THIRA report said the danger represented by the snowmelt depends in great part on whether it melts gradually or there is sudden warming or, worse, sudden warming coupled with thunderstorms.

"We are highly concerned that if we have a higher than average snowmelt, the amount of debris in the streams and channels of Boulder County will affect runoff negatively with debris dams forming and releasing, potentially creating flash flooding and damage," the report says.

Unlike thunderstorms, which allow time for officials to issue warnings, flooding from debris flows can be sudden, the report said.

"Detection will not occur until someone notices the presence of the dam or it releases and a surge of water is smashing through homes or washing away roads," the report says. "This gives very little reflex time to assess the situation (and) notify proper authorities of the hazardous condition and little time to warn the public to take action."

Debris dams that break in the canyons can have impacts downstream.

"Those things that happen upstream in the canyons could potentially cause impacts in the city, so if we have a rockslide or a debris dam that happens in the county, and water ponds behind that and it breaks loose, we could see surges in flow," Arthur said. "So we're obviously working closely with the county to identify and address areas where that might be an issue."

Boulder Creek in the city is much wider and has a much larger capacity than its tributaries in the canyons, so those flows are less likely to cause serious damage in the city, Arthur said. However, they can still represent safety hazards.

"It might not be a huge issue, but if you're on the bike path and the water comes up a foot out of nowhere, that can be of concern," Arthur said.

During September's floods, some of the worst damage occurred along creeks that are slated for flood mitigation projects but where the work had not occurred yet, such as Fourmile Canyon Creek and Twomile Creek.

"The creeks that we had done flood mitigation work in held up pretty well. It's the little creeks where we have plans in place but haven't been able to do the work yet, where we had the biggest damages," Bauer said. "That's where we'll be keeping a close eye."

Arthur said public works crews and emergency managers need the eyes and ears of the public to find hazards in time. The Inquire Boulder mobile app allows people to take pictures of debris in waterways and send it to the city, which will help officials prioritize a response depending on how much hazard the debris represents.

The risk of landslides will be higher this year, and if people see evidence of moving ground on open space trails — a tree or utility pole that is tilted when the day before it was straight, new cracks in trails or hillsides — they should call 911, city officials said.

City officials urged everyone in Boulder to consider getting flood insurance, even if they don't live in a floodplain. Anyone can buy flood insurance, and Boulder residents get a 25 percent discount off the base rate due to the flood mitigation projects the city has undertaken. People outside the floodplain pay lower rates, as well.

However, flood insurance doesn't go into effect until 30 days after the date of purchase, so residents should consider buying now in preparation for this flood season.

Arthur said Boulder residents also should consider buying sewer back-up insurance. It's not included in many homeowners' policies, but it can be purchased as an additional policy. Homeowners also can talk to a plumber about installing a check valve to prevent sewer back-ups.

Residents should get prepared now for possible future flooding because there might not be time later, Arthur said.

"What happened in September played out in slow motion," he said. "Our real risk here is flash flooding."

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